1. Field of the Invention
The present invention relates generally to the packaging of integrated circuits and other semiconductor products in plastic packages having a plurality of exposed metal leads. More particularly, the invention relates to an improved method for solder coating the leads to provide protection and allow for soldering the leads to a printed circuit board.
In packaging semiconductor devices, the integrated circuit (usually referred to as a chip or die) is connected to a plurality of metal leads, typically by either wire bonding techniques or tape automated bonding. After such connection, the die is encapsulated in either a plastic or ceramic package which protects the die and defines what will be referred to hereinafter as the semiconductor package. The present invention relates particularly to plastic semiconductor packages.
The metal leads, which are typically copper or a copper alloy project from the semiconductor package and may be connected to other circuitry by conventional soldering techniques. Copper, however, cannot be directly soldered and it is necessary to first coat the leads with a thin layer of tin or a tin/lead alloy to provide solderability.
Heretofore, two primary methods for coating the metal leads have been employed, both of which are applied to the semiconductor package after encapsulation is complete. The first method relies on dipping the semiconductor package in molten tin or tin/lead alloy for a brief period, typically 1 to 10 seconds. While this method is reasonably effective, the high temperature of the molten metal, typically from about 230.degree. C. to 260.degree. C., places severe thermal stress on the package and can cause gaps around the leads to develop in plastic package. Moreover, the solder coat applied to the leads often has a non-uniform thickness which can interfere subsequent manipulation of the device, particularly automatic insertion onto printed circuit boards.
The other method for coating a solder layer onto the metal leads relies on electroplating in a suitable plating bath. To accomplish such electroplating, the packages are first cleaned, typically with a strong mineral acid such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and the like. The plating electrolytes utilized typically consisted of corrosive acids, and tin/lead salts. For example, when coating pure tin, sulfuric acid based electrolytes were used. For coating tin/lead alloys, fluoboric acid based electrolytes were used. Alternatively, alkane/alkanol sulfonic acid based electrolytes were used.
The present invention addresses a problem which has been recognized by the inventors herein. When electroplating solder on plastic semiconductor packages, severe corrosion of the aluminum metallization on the semiconductor die can result if the package has small gaps between the plastic encapsulation material and the metal leads. Such gaps allow the corrosive cleaning solution and plating electrolyte to penetrate the plastic body of the semiconductor package. Once the corrosive materials have entered, subsequent rinsing and cleaning of the packages are often not effective to remove it. Even very small amounts of corrosive material left on the aluminum metallization for long periods of time can result in corrosion of the aluminum and present potential failure problems for the package.
It would therefore be desirable to provide methods for solder plating the metal leads of plastic semiconductor device packages in such a manner that corrosion of the metallization layers is largely avoided.
2. Description of the Prior Art
U.S. Pat. No. 4,163,700 discloses an electroplating bath for tin or tin alloys which includes tin salts, such as tin citrate, dissolved in a citric acid solution. The plating bath further includes a hydroxycarboxylic acid (other than citric acid or citrate) and/or a dibasic carboxylic acid. LeaRonal, Inc., Freeport, N.Y., sell an electroplating system for plating tin and/or tin alloys on various metals. The system is sold under the trade name "Solder On SG" and comprises tin and lead salts in a sulfonic acid based system. A sequestering agent (Solder On SG Make-up) of unidentified composition is included in the electroplating system. LeaRonal, Inc. recommends that their system be used on ceramic semiconductor packages, where the metal leads are cleaned with a strong mineral acid, such as sulfuric acid, prior to electroplating.